1. Field of the Invention
The present invention relates to a magnetic refrigeration device and a magnetic refrigeration method which can simplify the driving mechanism by conducting the heat transfer utilizing the solid heat conduction not requiring a liquid medium or a gaseous medium for the heat transfer.
2. Description of the Related Art
When the intensity of magnetic field to be applied is changed for a certain magnetic material, the temperature of the magnetic material also changes, which is called as a “magneto-caloric effect”. Physically, the degree of freedom of magnetic spins (electrons bearing the magnetic property) in the magnetic material is changed with the change of the external magnetic field so that the entropy of the magnetic spins is changed. In this case, the rapid energy exchange between the electrons and the lattice in the magnetic material occurs so as to change the temperature of the magnetic material which is concerned with the lattice vibration. The refrigerating operation based on the magneto-caloric effect as above-mentioned are called as a “magnetic refrigeration”.
At present, a gas compressing and expanding cycle refrigeration technique is widely used for practical application in daily life such as a refrigerator, a freezer and an air conditioning. However, in the refrigeration technique using the gas compressing and expanding cycle such as a chlorofluorocarbon (CFC) or hydrochlorofluorocarbon (HCFC) or hydrofluorocarbon (HFC) gas, there are some problems relating to the ozone layer destruction or the global warming caused by the environmental exhaust of those gasses.
Therefore, it is desired that a clean refrigeration technique with no harm gas medium and high efficiency without the environmental destruction caused by the exhaust of working gas is realized. Recently, the magnetic refrigeration technique near room temperature region is intensely researched and developed which is expected as an environment-friendly refrigeration.
As the magnetic refrigeration technique, the AMR (Active Magnetic Regenerative Refrigeration) system is proposed by “Barclay” in US (refer to U.S. Pat. No. 4,332,135). The AMR system is considered in view of the fact that the entropy of the lattice part is relatively larger than the entropy of the electronic part near room temperature range due to the thermal disturbance so that the temperature change of the magnetic material originated from the magneto-caloric effect becomes small. With the AMR system, therefore, the lattice entropy, which is considered as a problem for the magnetic refrigeration in high temperature range, is positively utilized. Namely, with the AMR system, the magnetic material works as not the magnetic refrigerant but also the magnetic regenerator in the magnetic refrigerating operation.
The AMR system is characterized in that the heat transfer is controlled in the magnetic refrigerant so as to a temperature gradient is generated effectively in the magnetic refrigerant by utilizing both of the works of magnetic refrigeration and the regeneration. Concretely, the magnetic refrigerant is divided into a plurality of pieces which are to be arranged so that the direct heat transfer between the pieces can be suppressed, ie; the heat is transferred only through the small contact area of the adjacent pieces. In this case, the coldness generated at each piece is transferred to the other piece mainly via the heat transfer medium (liquid or gas) by the heat exchange between the magnetic refrigerant and the heat transfer medium.
In this case, it is required the flow path of the heat transfer medium through the magnetic refrigerant is secured and the sufficient heat exchange between the magnetic refrigerant and the heat transfer medium is conducted. Therefore, the magnetic refrigerant should be configured so as to have the shape with a large specific surface area and securing the sufficient flow path of the heat transfer medium. Therefore, it is appropriate that the magnetic refrigerant shaped in honeycomb by folding the processed plate in cornice and packed into a magnetic refrigerating container (AMR bed). It is also appropriate that the magnetic refrigerant shaped as a mesh-like laminated body and packed into an AMR bed, or that the magnetic refrigerant processed in particle of spherical shape and packed into an AMR bed.
Zimm, Gschneidner and Pecharsky made the prototype of the AMR system and realized the continuous steady operation of the magnetic refrigerating cycle near room temperature range (region) (1997). In this case, gadolinium (Gd) metal was employed as the magnetic refrigerant which is shaped into spherical particles with the size of about 0.3 mm in diameter and packed into an AMR bed.
Since the adjacent spherical particles of Gd are point-contacted with one another, the mutual heat conduction between the particles can be allowed at low level. The inlet and the outlet of the heat transfer medium are provided at both ends of the AMR bed so that the heat transfer medium can be introduced into and discharged from the AMR bed. As the heat transfer medium, water or a mixture of water and ethanol was employed. The AMR bed was put into place the inside of coil of superconducting solenoid magnet (SM; superconducting solenoid magnet). The AMR can be displaced with reciprocating motion upward and downward along the axis of the SM coil, and inside and outside of the bore of SM coil so that the intensity of the magnetic field to be applied to the magnetic refrigerant in the AMR bed can be varied by displace the location of the AMR bed.
The AMR cycle of refrigeration can be conducted as follows: (1) The AMR bed is put into the bore of the SM coil, and the magnetic field is applied to the magnetic refrigerant thereby the magnetic refrigerant heat up. (2) The heat transfer medium is flowed through the magnetic refrigerant in the AMR bed from the one end to the other end of the AMR bed and transfer the thus generated hotness. (3) The AMR bed is removed from the bore of the SM coil to remove the magnetic field applied to the magnetic refrigerant, thereby the magnetic refrigerant cool down. (4) The heat transfer medium is flowed from the other end to the one end of the AMR bed (the direction opposite to the direction in the Step (2)) and transfer the coldness.
By repeating the heat cycle of Steps (1)-(4), a temperature gradient can be generated in the magnetic refrigerant packed into the AMR bed. To begin with the magnetic refrigerant heat up by applying a magnetic field to the refrigerant, and the hotness is transferred from the magnetic refrigerant to the heat transfer medium. Then, the hotness transports with the heat transfer medium by flowing forward direction and then, hotness transferred from the heat transfer medium to the magnetic refrigerant. Likewise, the magnetic refrigerant cool down by removing the magnetic field from the refrigerant, and the coldness is transferred from the heat transfer medium to the magnetic refrigerant. In this case, the coldness transports with the heat transfer medium by flowing backward direction and then, coldness transferred from the heat transfer medium to the magnetic refrigerant.
Namely, the intended heat transfer is mainly conducted by the indirect heat conduction via the heat transfer medium, not by the direct heat conduction via the point contact between the particles of the magnetic refrigerant. In addition, since each particle stores the corresponding heat generated by the heat cycle, the difference in temperature between the adjacent particles is generated so that the temperature gradient is generated in the direction along the heat flow by the heat transfer medium. In the steady state, therefore, a large difference in temperature can be generated between both ends of the AMR bed.
According to Zimm et al., the thermal difference is generated at both ends of the AMR bed by ΔT=about 30° C. by changing the intensity of the magnetic field from zero to 5T with the superconducting magnet near room temperature range. Then, a high refrigerating efficiency of COP=15 (not containing the input power for the SM) can be realized under the condition of ΔT=about 13° C. With the conventional technique using gas compressing and expanding cycle of Freon gas, e.g., in a refrigerator of household use, the refrigerating efficiency of only COP=1-3 can be realized.
[Reference 1]U.S. Pat. No. 4,332,135[Reference 2]U.S. Pat. No. 5,743,095[Reference 3]C. Zimm, et al., Advances in CryogenericEngineering, Vol. 43 (1998), p. 1759(Consideration of size and simplificationin magnet)
In the above-described embodiment, however, the superconducting magnet (SM) is employed in order to apply the high magnetic field, e.g., 5T to the magnetic refrigerating working material. Since operating the superconducting magnet (SM) requires the extreme low temperature of about 10K, which needs a liquid helium or a refrigerator for generating extreme low temperature. Therefore, the magnetic refrigeration system is grown in size.
An electromagnet (EM) may be employed instead of the superconducting magnet (SM). With the electromagnet (EM), in order to generate a magnetic field in the intensity of 1T or over, a large current must be applied to the electromagnet (EM) so as to require the water cooling system for removing the Joule heat generation from the electromagnet (EM). Therefore, the magnetic refrigeration system becomes complicated, grows in size and requires high operation cost in the same manner as the superconducting magnet.
(Problem in Use of Heat Transfer Medium in the AMR)
In order to realize the AMR system in the above-described embodiment, the magnetic refrigerant is processed in spherical particle so that the thus obtained particles can be packed into the AMR bed and the heat transfer medium is made of a mixture of water and ethanol. Namely, the heat is transferred from one end to the other end of AMR bed mainly by flowing the liquid transfer medium through the heat exchange between the solid particles and the liquid transfer medium, not by the direct heat conduction of the solid particles. Therefore, the liquid or gaseous heat transfer medium is required in addition to the solid magnetic refrigerant. Moreover, the driving mechanism to move the heat transfer medium is also required.
In the case that the magnetic refrigeration is conducted near room temperature region, the liquid heat transfer medium is better than the gaseous medium in view of the heat capacity. The wet type magnetic refrigerating device using the liquid heat transfer medium has some disadvantages such as handling and complicated design.
(Problem in Use of Heat Transfer Medium in the AMR)
In the case that the heat transfer is conducted by the heat transfer medium, the magnetic refrigerant is subjected to the pressure shock of the flow of the heat transfer medium. In the case that the magnetic refrigerant is brittle, the magnetic refrigerant may be cracked by the repeated pressure shock. In this case, fine powders may be generated due to the crack of the magnetic refrigerant so as to disturb the flow of the heat transfer medium and thus, to deteriorate the performance of the magnetic refrigerating system.
Particularly, in the case that the spherical particles of the magnetic refrigerant are packed into the AMR bed, the fine powders may be generated by the collision between the particles and the crash of the particles against the inner wall of the AMR bed. In this case, the flow path in the packed particles can not be maintained so as to increase the pressure loss of the heat transfer medium and thus, to deteriorate the refrigerating performance of the magnetic refrigerating system. Even though the magnetic refrigerant can exhibit a large magneto-caloric effect, it becomes difficult to use the magnetic refrigerant if the magnetic refrigerant can not exhibit enough mechanical strength against the repeated pressure shock.
(Problem Relating to Effective Use of Magnetic Field)
In order to mitigate the growth in size of the magnet system, it is preferable to employ a permanent magnet. However, the permanent magnet can generate a magnetic field in intensity much smaller than the superconducting magnet. In the case that the magnetic refrigerant is packed into the AMR bed and the magnetic field is applied to the magnetic refrigerant by the permanent magnet, the affect of the demagnetization field is largely, in comparison with the coil-type magnet of superconducting magnet or the electromagnet.
In this point of view, it is desired in view of thermal design that appropriate for the AMR system, the AMR bed to accommodate the magnetic refrigerant is shaped in column such as cylindrical column or rectangular column so that the long direction of the AMR bed can correspond to the heat flow. The reason is that in the AMR system, the temperature gradient is formed in the direction of the heat flow utilizing the heat storage of the magnetic refrigerant so as to generate the temperature difference at both ends of the AMR bed.
Generally, when the magnetic field is applied to a magnetic material, the thus generated demagnetization field strongly depends on the geometrical shape of the magnetic material. In the case that the magnetic material is shaped in column, the demagnetization field becomes minimum when the magnetic field is applied along the long direction of the magnetic material and the demagnetization field becomes maximum when the magnetic field is applied along the width direction of the magnetic material. In the use of the superconducting magnet or the electromagnet, the magnetic field is generated in the bore space of the magnet coil along the center axis of the magnet coil. Therefore, when the columnar AMR bed is disposed in the bore space of the coil, the magnetic field can be applied to the AMR bed along the long direction (the center axis of the bore space of the coil) by increasing the aspect ratio of the AMR bed because the aspect ratio of the height and the diameter of the AMR bed can be controlled freely. In this case, therefore, the AMR bed is unlikely to suffer from the demagnetization field.
On the other hand, the permanent magnet can be configured as a face-type U-shaped magnet with a magnetic yoke or a Halbach-type magnet. With the U-shaped magnet, the magnetic field to be generated depends on the ratio of the magnet volume to the space gap volume. In order to increase the intensity of the magnetic field, it is required to narrow the space gap. In the use of the U-shaped magnet, when the columnar AMR bed is disposed in the space gap, the magnetic field is applied to the AMR bed along the width direction.
With the Halbach-type magnet, the magnetic field is generated in the bore space of the magnet along the width direction of the bore space. As a result, when the columnar AMR bed is disposed in the bore space, the magnetic field is applied to the AMR bed along the width direction. In this way, even though the Halbach-type magnet is employed, the thus generated magnetic field is applied along the width direction of the AMR bed. Therefore, when the permanent magnet is employed as a magnet in the AMR system, there are some disadvantages such as the large reduction of the intensity of the magnetic field, which is to be applied to the magnetic refrigerant accommodated in the AMR bed, originated inherently from the use of the permanent magnet and from the large demagnetization field due to the geometrical shape of the AMR bed which is restricted in design.